The present invention relates to a method of sulfonating an internal olefin, thereby preparing a sulfonate thereof. Particularly it relates to a method of preparing a light-colored internal olefin sulfonate at a high yield. The term "sulfonating", as employed hereinafter in the specification and in the claims, is used sometimes in its generic sense as applying both to true sulfonating and to sulfating, and sometimes in its specific sense, that is, to true sulfonating.
As is generally known, internal olefins are low in reactivity to a sulfonating agent compared with .alpha.-olefins. Therefore, on the occasion of sulfonating an internal olefin with SO.sub.3 gas, for instance, if a light colored sulfonation product is aimed at, the rate of reaction will markedly lower, and yet, raising of the rate of reaction will extremely deteriorate the color tone of the sulfonation product. Such a tendency is in evidence with respect to .alpha.-olefins too. However, in the case of .alpha.-olefins, it is possible to obtain a sulfonation product having a light color and containing a low percentage of unreacted olefin by adoption of the so-called two step-sulfonation process which comprises conducting sulfonation with SO.sub.3 gas under relatively mild conditions and subsequently conducting sulfonation with concentrated sulfuric acid. But, in the case where this two step-sulfonation process is applied to an internal olefin, because of the poor reactivity of the internal olefin per se, the sulfonation mixture obtained through the former step of sulfonation is still rich in unreacted olefin and there is brought about a situation that this unreacted olefin would get polymerized in the course of the latter step of sulfonation, thereby rendering it impossible to obtain the desired sulfonation product at a high yield.
The present inventors have made a series of studies with a view to settling the foregoing problems peculiar to internal olefins and have come to a finding that on the occasion of effecting sulfonation of an internal olefin through the two step sulfonation process, it is possible to prepare a light-colored internal olefin sulfonate at a high yield when the former step of sulfonation is performed by applying a relatively high ratio of SO.sub.3 to the olefin and the latter step of sulfonation is performed in the presence of anhydrous sodium sulfate.
The art of effecting sulfonation reaction in the presence of some inorganic sulfate like sodium sulfate as such has been introduced through the specification for U.S. Pat. No. 4,021,406 disclosing a process for manufacturing sulfo-fatty acid esters. But, the object of the use of inorganic sulfates in this prior art is to prevent the cleavage of ester bond and concurrently avoid the degeneration of color tone of the product. That is, this prior art has no ideas of preventing the polymerization of unreacted olefins by adding inorganic sulfates thereto.